This application relates to methods and devices for generation of oscillating signals, and more specifically, to generation of oscillating signals by using opto-electronic oscillators.
Oscillating signals can be generated by using various types of oscillators having energy storage elements. The quality factor Q, or the energy storage time, of an energy storage element can determine the spectral linewidth of the respective oscillating signal. Increasing the quality factor Q or the energy storage time can reduce the spectral linewidth of the oscillating signal and hence improve the signal""s spectral purity.
Spectrally pure radio frequency (RF) oscillators can be used for generating, tracking, cleaning, amplifying, and distributing RF carriers. Such RF carriers can have important applications in communication, broadcasting, and receiving systems in the radio frequency spectral range. In particular, voltage-controlled RF oscillators with phase-locked loops can be used for, among others, clock recovery, carrier recovery, signal modulation and demodulation, and frequency synthesizing.
RF oscillators can be constructed by using both electronic and optical components to form opto-electronic oscillators (xe2x80x9cOEOsxe2x80x9d). See, e.g., U.S. Pat. Nos. 5,723,856 to Yao and Maleki and U.S. Pat. No. 5,777,778 to Yao. Such an OEO includes an electrically controllable optical modulator and at least one active opto-electronic feedback loop that comprises an optical part and an electrical part interconnected by a photodetector. The opto-electronic feedback loop receives the modulated optical output from the modulator and converted it into an electrical signal to control the modulator. The loop produces a desired delay and feeds the electrical signal in phase to the modulator to generate and sustain both optical modulation and electrical oscillation in radio frequency spectrum when the total loop gain of the active opto-electronic loop and any other additional feedback loops exceeds the total loss.
OEOs use optical modulation to produce oscillations in frequency spectral ranges that are outside the optical spectrum, such as in RF and microwave frequencies. The generated oscillating signals are tunable in frequencies and can have narrow spectral linewidths and low phase noise in comparison with the signals produced by other RF and microwaves oscillators. Notably, the OEOs are optical and electronic hybrid devices and thus can be used in optical communication devices and systems.
A variety of OEOs can be constructed based on the above principles to achieve certain operating characteristics and advantages. For example, another type of OEOs is coupled opto-electronic oscillators (xe2x80x9cCOEOsxe2x80x9d) described in U.S. Pat. No.5,929,430 to Yao and Maleki. Such a COEO directly couples a laser oscillation in an optical feedback loop to an electrical oscillation in an opto-electronic feedback loop. Opto-electronic oscillators can also be implemented by having at least one active opto-electronic feedback loop that generates an electrical modulation signal based on the stimulated Brillouin scattering. U.S. Pat. No. 5,917,179 to Yao. Such a Brillouin OEO includes a Brillouin optical medium in the feedback loop and uses the natural narrow linewidth of the Brillouin scattering to select a single oscillating mode.
The present disclosure includes opto-electronic oscillators that implement at least one high-Q optical resonator in an electrically controllable feedback loop. An electro-optical modulator is provided to modulate an optical signal in response to at least one electrical control signal. At least one opto-electronic feedback loop, having an optical part and an electrical part, is coupled to the electro-optical modulator to produce the electrical control signal as a positive feedback. The electrical part of the feedback loop converts a portion of the modulated optical signal that is coupled to the optical part of the feedback loop into an electrical signal and feeds at least a portion of it as the electrical control signal to the electro-optical modulator.
The high-Q optical resonator may be disposed in the optical part of the opto-electronic feedback loop or in another optical feedback loop coupled to the opto-electronic feedback loop, to provide a sufficiently long energy storage time and hence to produce an oscillation of a narrow linewidth and low phase noise. The mode spacing of the optical resonator is equal to one mode spacing, or a multiplicity of the mode spacing, of the opto-electronic feedback loop. In addition, the oscillating frequency of the OEO is equal to one mode spacing or a multiple of the mode spacing of the optical resonator.
The optical resonator may be implemented in a number of configurations, including, e.g., a Fabry-Perot resonator, a fiber ring resonator, and a microsphere resonator operating in whispering-gallery modes. These and other optical resonator configurations can reduce the physical size of the OEOs and allow integration of an OEO with other photonic devices and components in a compact package such as a single semiconductor chip.